Titanium nitride layers have found utility in the manufacture of semiconductor devices, particularly as barrier layers between a semiconductor substrate, e.g., a silicon wafer, and overlying conductive layers from which contacts are made. For example, a layer of titanium may be sputter deposited and annealed in nitrogen to form a titanium nitride layer that prevents an after deposited aluminum layer from spiking through the layer and directly contacting the underlying silicon substrate, causing a short. Titanium nitride has a low bulk resistivity and makes good ohmic contact to conductive layers such as aluminum. It also has good adhesion properties.
The preferred method of forming titanium nitride barrier layers up till recently has been to sputter titanium metal in a nitrogen environment, thereby depositing titanium nitride onto a silicon wafer surface, or into contact openings in the silicon surface. This method of deposition can be used at high temperatures, when the titanium metal reacts with the underlying silicon surface to form an ohmic contact of titanium silicide on the silicon, followed by formation of the titanium nitride barrier layer. Thus the titanium silicide and titanium nitride barrier layers can be formed in a single step. The resistivity of these films is very low, about 36 .mu.ohms-cm.
However, as devices become ever smaller and more devices are made on a single silicon wafer, requiring a higher density of devices, it is more difficult to sputter deposit uniformly thick layers of titanium nitride over the bottom and sidewalls of an opening, particularly as the aspect ratio of the opening (ratio of depth to width) becomes higher; and to cover very narrow lines without leaving voids at the base corners. The bottom step coverage for 0.6 micron wide lines, or deposition into a 1.75:1 aspect ratio opening, is only about 46%.
Thus more recently a chemical vapor deposition method has been employed which deposits more conformal layers of titanium nitride. Sandhu et al, see U.S. Pat. No. 5,246,881, have disclosed the deposition of titanium nitride from a metallo-organo titanium compound having the formula Ti(NR.sub.2).sub.4 wherein R is an alkyl group. These metallo-organo titanium compounds can be decomposed with activated species of halogen, hydrogen or ammonia for example, at relatively low temperatures of 200-600.degree. C. at a vacuum pressure of about 0.1-100 Torr, to deposit highly conformal titanium nitride layers. This method deposits titanium nitride at good deposition rates, with excellent conformality and step coverage (the step coverage is about 90% over a 0.6 micron wide line or into an opening having an aspect ratio of 1.75:1).
However, when aluminum is deposited over high aspect ratio openings in silicon, high aluminum deposition temperatures, e.g., at or above the flow temperature of aluminum, must be used so that the aluminum will uniformly fill the openings and form a planarized layer. However, when high aluminum deposition temperatures are used, the titanium nitride barrier layers are inadequate to prevent aluminum spiking through the titanium nitride layer.
Thus a method was sought to improve the barrier properties of titanium nitride layers to prevent aluminum spiking when the substrate is heated to temperatures above about 400-450.degree. C., near the flow temperature of aluminum.
It is also known that the addition of oxygen to titanium nitride barrier layers improves their barrier properties. Such a process is known for sputter deposition processes, when, after a high temperature anneal to form a titanium silicide contact layer. a low temperature anneal is carried out in the presence of a small amount of oxygen. This two-step anneal must be carried out sequentially at high vacuum, without removing the substrate from a vacuum environment. The high vacuum required for sputter deposition is from about 10.sup.-6 to 10.sup.-9 millitorr.
However, the chemical vapor deposition process is carried out at much higher pressures of about 0.1 to 100 Torr. Oxygen cannot be added to the deposition chamber, because the chamber contains large amounts of carbon-containing radicals, which would react to form silicon oxide. Contamination of the titanium nitride films would result, increasing the bulk resistivity of the films to unacceptably high levels. Further, these titanium nitride films are unstable and their resistivity increases over time. Sandhu et al added a Lewis base to the metallo-organo titanium precursor compound to reduce the resistivity of the film, such as a Lewis base of chlorine, ammonia or nitrogen trifluoride, but these Lewis bases introduce other contaminants into the titanium nitride films.
Thus a method of post treating titanium nitride films deposited by chemical vapor deposition of organo-metallo titanium compounds, that will stabilize the films to elevated temperatures has been sought.